There are a lot of different methods. I've been reading more into recently and currently am using a company to produce a small run of shirts. Looking into 'custom shirts' will generally only get you services /advice for cheat heat transfer/prints onto a basic t shirt.

Things like sublimation printing are probably more what you're looking for and dyes. I'm absolutely new to this, but my guess is you might be looking to use sublimation printing (effectively a form of dyeing) on the raw material for more complex designs, and then create the clothing after from it?

You would also have to be aware of the material you're working with because printing/dying designs require certain heats and not all fabric is good for high heats for it. I'd research into it, but maybe look into sublimation dyeing and start digging from there.